The present invention generally relates to evaporative coolers and more specifically to a heat exchange apparatus such as a closed-loop cooling tower or an evaporative condenser.
Evaporative coolers are commonly employed which include indirect and direct heat exchange sections. An evaporative liquid, generally water, is distributed across an indirect heat exchange section. The indirect heat exchange section is typically comprised of a series of individual, enclosed circuits or loops for conducting a fluid stream which is to be heat treated, that is, to be cooled. When the evaporative cooler is used as a closed-loop cooling tower or evaporative condenser, heat is indirectly transferred from the fluid stream to sensibly heat the surrounding film of evaporative liquid flowing over the enclosed circuits thereby warming the evaporative liquid. Oftentimes these enclosed circuits are a series of tubes or assembly of coils which may be circular in cross section or which may have non-circular cross sections, such as those disclosed in U.S. Pat. No. 4,755,331, the disclosure of which is incorporated herein by reference.
Heat absorbed by the evaporative liquid is directly transferred to an air stream in a direct evaporative heat exchange section. In the direct evaporative heat exchange section the evaporative liquid is directed onto a solid surface area, commonly referred to as wet deck fill and a small portion of the liquid evaporates, thereby cooling the remaining portion. This fill may comprise a variety of constructions such as wooden slats, corrugated metal sheets, stacks of formed plastic sheets, etc. For example, a certain type of fill is disclosed in U.S. Pat. No. 5,124,087, the disclosure of which is incorporated herein by reference.
Over the past 50 years, improvements in the technology of wet deck fill have been tremendous. Wet deck fill has evolved into highly efficient sheets of multifaceted plastic that is much more efficient than the old splash fill, capable of low pressure drops and allows the temperature of the evaporative liquid leaving the fill to approach wet bulb temperatures.
In the earlier days of cooling tower wet deck fill development, the best technology was simply stacked wooden slats that caused the water to splash and turbulate the air flowing through. The object of wet deck fill is to expose as much of the water surface area as possible to as much air flow as possible for as long a time period as possible with a minimal resistance to air flow. The early cooling tower wet deck fills were very inefficient in this process. At that time it was common practice to place a heat transfer coil in the air and water stream without the use of any cooling tower wet deck fill. Wet deck fill had very little advantage over the geometry of tubes in the air stream with water splashing over it.
The invention of improved wet deck fills has caused more and more inventions that use combinations of fill and coil to do this type of cooling. As fill performance improved, inventors discovered the benefit of combining the two media. However, the prior art has emphasized the importance of air flow over (and through) the coil assembly which is coupled with the wet deck fill. In every case, this art still shows the coil with air flowing through it. The inventive efforts over the years have all been directed towards methods of easing or improving the flow of air through the heat transfer coil. Even with these improvements in coil design, the coils were limited in the amount of water that could be sprayed over the coil without choking off the flow of air. In some instances the flow of air had to be arranged in parallel with the flow of water to allow for the desired flow of air through the coil.
Typical evaporative coolers have included the coil of the indirect heat exchanger as part of the fill, either interspersed within the fill in the direct heat exchange section as disclosed in U.S. Pat. No. 3,012,416, or in separate sections, with both the direct and indirect sections relying, at least in part, on significant air flow therethrough for evaporative direct heat exchange to occur in both sections, such as disclosed in U.S. Pat. Nos. 5,435,382; 4,683,101; 5,724,828 and 4,112,027.
The evaporative liquid is typically recirculated through the evaporative cooler such that it passes from the indirect cooling section to the direct cooling section and back to the indirect cooling section in a continuous cycle with makeup liquid added to compensate for the liquid which has evaporated.
The present invention recognizes the advantages of developments in the art and combines those advantages in unique ways to achieve surprising and unexpected results.
Although all of the prior art teaches the logical idea that putting airflow through the coil will aid in the cooling process, Applicants have determined the surprising result that putting additional airflow through the coil only serves to decrease the performance of the wet deck fill and burden the air moving system with additional flow requirements, costing extra air moving horsepower. While it is not critical for Applicant""s invention that there be no air flow at all over the heat transfer coil, Applicants have discovered that the overall performance of the evaporative cooler is enhanced if the air flow over the heat transfer coil is minimized or avoided altogether.
By the present invention, the Applicants have maximized the efficiency of the wet deck fill by distributing the water to be cooled over a relatively larger plan area of a fill housing. This maximizes the amount of water surface area in contact with the airflow and minimizes the work required from the air moving device.
Applicants have made the discovery that when liquid is cascaded over the heat transfer coil of the indirect heat exchanger at very high (or concentrated) flow rates it has surprisingly high heat transfer coefficients or U-values.
Applicants have recognized and utilized the advantage of increasing the liquid load on the indirect heat transfer section (by amounts up to 8 to 16 gallons per minute per square footxe2x80x9422.74 to 45.48 liters per minute per square meter) while avoiding the disadvantage of increasing the liquid load on the wet deck fill, by providing a smaller plan area for the indirect heat transfer section coil than for the fill and concentrating the liquid flow as it moves from the fill to the coil.
In addition they discovered that the U-value can be increased in two ways, by providing a higher liquid load at the heat transfer coil and/or by increasing the velocity of liquid flow onto or through the heat transfer coil section.
The applicants discovered the surprising results that by not burdening the coil with a cooling airstream they were free to highly concentrate the flow over the coil and to position the coil wherever they wanted without regard to the geometry of the airflow. Also, they were able to take advantage of the increased velocity of the falling water to further enhance the heat transfer coefficient of the coil.
In summary, in an embodiment, the applicants have separated and made more efficient, each heat transfer section although every prior inventor had combined the sections to one degree or another in attempts to achieve the most efficient device. The applicants"" invention separates the fill from the coil so the fill can be used to it""s maximum efficiency and the coil can be used to it""s maximum efficiency.
Specifically, in an embodiment, an evaporative cooler embodying the principles of the present invention includes a liquid distributor for distributing an evaporative liquid (sometimes referred to simply as water) onto a gas and liquid contact body (the wet deck fill) having a surface for receiving the liquid and occupying a first plan area for receiving liquid from the liquid distributor over the surface substantially throughout the first plan area. An air moving device is arranged to generate a flow of air and the body surface is arranged in the flow of air, the flow of air causing a small portion of the liquid received by the body to evaporate, thereby cooling the remaining non-evaporated portion of the liquid. A heat transfer working fluid conduit (the heat transfer coil) is positioned substantially outside of the flow of air and has a second plan area dimensioned smaller than the first plan area. The heat transfer coil has a surface arranged to receive substantially all of the cooled liquid from the body. A liquid concentrator is arranged between the body and the heat transfer coil to concentrate the cooled liquid from the first plan area into the second plan area. The cooled liquid, as it falls over the surfaces of the heat transfer coil, is sensibly re-heated as heat is withdrawn from the working fluid circulating inside the conduit, to cool the working fluid. A liquid collector receives substantially all of the falling, heated liquid from the heat transfer working fluid conduit. A liquid recirculating mechanism returns the heated liquid to the liquid distributor for a repeat of the cycle.
In an embodiment of the invention, the evaporative cooler comprises a liquid distributor and a body for receiving liquid from the liquid distributor. An air moving device is arranged to generate a flow of air over the body, the flow of air causing a small portion of the liquid received by the body to evaporate, thereby cooling the remaining non-evaporated portion of the liquid. A heat transfer working fluid conduit is arranged to receive substantially all of the cooled liquid from the body. A flow accelerator is positioned between the body and the heat transfer working fluid conduit to accelerate a flow velocity of the cooled liquid by at least 9.5 feet per second (2.9 meters per second) before contacting a surface of the heat transfer working fluid conduit. The cooled liquid, as it falls over the surfaces of the heat transfer working fluid conduit, is sensibly heated as it cools the working fluid circulating inside the conduit. A liquid collector is positioned to receive substantially all of the heated liquid from the surface of the heat transfer working fluid conduit. A liquid recirculating mechanism is provided to return the heated (or collected) liquid to the liquid distributor.
In an embodiment of the invention, a method is provided of cooling a working fluid comprising the step of dispensing a liquid onto a surface of a body wherein the body occupies a first plan area. The air is flowed over the body to effect an evaporation of a portion of the liquid thereby cooling the remaining portion. The remaining cooled portion of the liquid is dispensed and concentrated onto a surface of a heat transfer working fluid conduit wherein the heat transfer working fluid conduit occupies a second plan area smaller than the first plan area and is maintained in an area substantially free of an air flow. The evaporatively cooled fluid is flowed over and around the heat transfer working fluid conduit to transfer heat between the working fluid and the evaporatively cooled liquid. In this process the evaporatively cooled liquid is heated and the working fluid inside the conduit is cooled. The heated liquid is collected from the exterior surface of the heat transfer working fluid conduit and recirculated onto the body.
An advantage provided by an embodiment of the present invention is that when the coil is spaced below the wet deck fill in a factory built module, the center of gravity of the module is lowered, which improves the transportability of the module. Once such a construction is in place, whether factory built or built on site, the lower center of gravity provides advantages related to seismic loading considerations, steel loading considerations and wind loading considerations.
In embodiments of the present invention which have the coil spaced from the wet deck fill, all six sides of the coil are readily accessible, at ground level, which allows for ease of access for inspection or cleaning of the coil.
In embodiments of the present invention where the coil is substantially or completely outside of the airstream flowing through the cooler, there is less of a chance for scale to form on the coil from the evaporative process. Such scale could otherwise negatively impact on heat transfer through the coil in that it acts as a heat insulator, reducing the heat transfer effectiveness through the coil walls.
Also in embodiments of the present invention where the coil is substantially or completely outside of the airstream flowing through the cooler, the air is protected against contamination from air borne dirt and debris, as well as sunlight passing through louvers or other openings. Also, in some situations, unintentional heat transfer occurs at a conventional airstream exposed coil, which would be avoided in such embodiments where the coil is located substantially or completely outside of the airstream.